FIG. 6 shows an example of a prior art system for measuring low current. In FIG. 6, a core wire of a reed relay 7 (signal line 71 brought out from reed switch 74) is connected to a signal output terminal (force terminal 21) of a system which contains a current source and which has an active guard. External conductive casing 72 of reed relay 7 is connected to a guard terminal 22 of a system containing a current source 2, through a switch 81, and to a ground terminal (guard terminal 12) of a picoammeter 1 (pA in FIG. 6). The other core wire of reed relay 7 (i.e., signal line 73) is connected to the signal input terminal (sense terminal 11) of picoammeter 1 and to the ground terminal (guard terminal 12) through switches 82 and 83.
The drive coil of reed relay 7 is omitted in FIG. 6.
In this measurement system, when current flowing through force terminal 2L from current source 2 is not being measured (i.e., during non-measurement period), the contact points of reed switch 74 and switches 81 and 82 are opened, and switch 83 is closed. When the current flowing through force terminal 21 from current source 2 is being measured (i.e., during measurement period), an opposite configuration is present: i.e., reed switch 74 and switches 81 and 82 are closed, and switch 83 is opened.
As mentioned above, signal wire 73 and conductive casing 72 are forced to the same potential in both the current non-measurement and the current measurement periods through the operation of reed switch 74 and switches 81-83.
In the measurement system of FIG. 6, however, there are cases in which the voltage which appears at force terminal 21 differs during the current non-measurement and current measurement periods. The operation in this case will be explained by referring to the equivalent circuit of the reed relay in FIG. 7. In FIG. 7, a1 and a2 are terminals which are connected, respectively, to force terminal 21 and guard terminal 22. Terminals b1 and b2 are connected respectively to sense terminal 11 and guard terminal 12 of picoammeter 1. K1 is the contact point of the reed switch, C1 the capacitance between signal lines 71 and 73, C22 is the capacitance between signal wire 71 and external casing 72, and C21 is the capacitance between signal wire 73 and external casing 72.
When a voltage appears at force terminal 21, that voltage charges signal line 71 and external casing 72 with a charge that is based on static capacitance C22.
When current is being measured, reed switch 74 and switches 81 and 82 are closed, while switch 83 is opened, as mentioned above. If at this time, there is only air in the capacitance space between signal wire 71 and conductive casing 72, the charging current flows instantly into picoammeter 1. However, in reed relay 7 shown in FIG. 6, reed switch 74 is within conductive casing 72, so that insulating parts 75 and 76 are interposed in the capacitance space. This produces the following problems.
Insulating parts 75 and 76 have dielectric absorption properties. The dielectric absorption due to insulating part 75 prevents an instantaneous discharge of the charge between signal wire 71 and conductive casing 72 (capacitance C22 in FIG. 7). Therefore, when the output current of force terminal 21 is measured by picoammeter 1, the current measurement cannot be performed until a suitable time passes after reed switch 74 is closed (i.e., there is a large measurement waiting time).
For example, in the case of a conventional reed relay 7, if it is assumed that a voltage of 100 V appears on the signal wire 71 (immediately before the current measurement), the value of the current due to the discharge of the charge between signal wire 71 and external conductor 72 frequently requires several tens of seconds to fall to the femto-ampere level.
This invention has the objective of providing a contact making and breaking device which (I) greatly improves the dielectric absorption properties of capacitance between a signal wire emanating from a reed switch and an external conductive casing, and (ii) greatly shortens (or reduces to zero) the measurement waiting time (the time until the erroneous current becomes small enough not to affect the measurement) when low current is measured.
The aforementioned problems are caused by:
(1) the fact that an insulating part is present which reduces the dielectric absorption property in the capacitance space (see C22 in FIG. 7) between the signal wire and the conductive casing of the reed relay that is connected to the system which contains a current source; and PA1 (2) the fact that, during the non-measurement period, the current due to the charge between the signal wire (connected to the system which contains a current source) and the external conductor (i.e., the charge based on the existence of the aforementioned insulating part) flows into the picoammeter as an error current when one tries to start a current measurement (when the reed relay is closed). PA1 (1) the aforementioned problems can be solved by covering the space between the signal wire and the external conductor with a tubular conductor, so that no insulating part exists in the capacitance space between the signal wire and the external conductor; and PA1 (2) since the aforementioned tubular conductor is placed in the external conductive casing, some of insulating parts are present in the capacitance space between the signal wire from the reed switch (the signal wire on the side connected to the system which contains a current source) and the tubular conductor. A closed circuit is thus formed, so that current due to the charge resulting from the insulating part does not flow into the picoammeter.